The present invention relates to a semiconductor storage device, and more specifically, to a semiconductor storage device including an insulator having a function of holding electric charges, a manufacturing method therefor, and portable electronic equipment.
As nonvolatile memories using insulators having a function of trapping electric charges, there are memories having a MONOS (Metal Oxide Nitride Oxide Semiconductor) structure (see, e.g., JP 05-145078 A). As shown in FIG. 24, a MONOS (memory element 99) has a field-effect transistor structure in which diffusion regions 902, 903 that are to be source/drain regions are formed in a semiconductor substrate 901 and a gate electrode 905 is formed above a channel region between these two diffusion regions with an ONO (Oxide Nitride Oxide) film 904 disposed between the gate electrode and the channel region. The ONO film 904 is structured such that a silicon nitride film 908 is interposed in between a bottom insulating film 906 and a top oxide film 907. The quantity of electric charges trapped by the silicon nitride film 908 changes a threshold value of the memory element 99, and detecting the difference in threshold value makes it possible to read stored information.
However, regarding a memory retention characteristic when the MONOS is in an erased state (in which holes are accumulated in the silicon nitride film 908 and the threshold value is low), it is known that the threshold value increases (the read current decreases) with a lapse of time. FIGS. 25 and 26 are graphs schematically showing time (t) dependence of a threshold value (Vt) and of a read current quantity (Id), respectively, after the MONOS is put in an erased state. Such characteristics of the MONOS cause reduction in memory window (difference in read current or difference in read threshold between program (i.e., write) operation and erase operation), thereby posing a major impediment to increase in read speed or miniaturization of the elements. Such reduction in read current with a lapse of time in the MONOS is a distinctive phenomenon particularly dominantly produced in nonvolatile memory elements using insulators having a function of trapping electric charges.